Two areas of interest for improving the speed and performance of semiconductor devices include strained silicon and silicon on insulator technologies. Strained silicon technology has been shown to enhance carrier mobility in both n and p-channel devices, and is being considered to improve the electron mobility and drift velocity in n-channel MOSFET's in CMOS technology. Silicon on insulator structures have been shown to reduce parasitic capacitances, and to increase switching speed of digital circuits and frequency in radio frequency (RF) circuits.
One technique for producing strained silicon involves growing silicon (Si) layers on relaxed silicon-germanium (SiGe) layers. A large mismatch in the cell structure between the Si and SiGe layers causes a pseudo-morphic layer of Si on relaxed SiGe to be under biaxial tensile strain. The biaxial strain modifies the band structure and enhances carrier transport in the Si layer. For example, a 1% strain of the silicon layer almost doubles the electron mobility. One method for forming the strained Si layer on the relaxed SiGe layer involves epitaxially growing the Si and SiGe layers using an ultra-high vacuum chemical vapor deposition (UHVCVD) process, and is a costly and complex process.
A proposed back end approach for straining silicon applies uniaxial strain to wafers/dies after the integrated circuit process is complete. The dies are thinned to membrane dimensions and then affixed to curved substrates to apply an in-plane, tensile strain after device manufacture. Another method for straining silicon forms voids in a transistor structure to produce a localized mechanical strain in the silicon.
One technique for fabricating a silicon-on-insulator structure involves a SIMOX (Separation by IMplantation of OXygen-Silicon-On-Insulator) process. The SIMOX process uses a very high dose and high energy oxygen implant followed by an oxide growth to form a deep and thick buried oxide (BOX) region. The SIMOX process typically forms a BOX region 3000 Å thick or thicker. The silicon layer over the BOX region is typically 1000 Å or thicker.
There is a need in the art to provide improved semiconductor structures, and to provide methods of forming improved semiconductor structures, that improve the speed and performance of semiconductor devices through increased mobility and decreased stray capacitive loading.